A political newcomer, McFadden was the front-runner after he was endorsed at the party convention in April. He argued he was the only candidate who could raise enough money to beat Franken. He criticized the first-term senator as being too close to Obama and for his support of the president's health care overhaul.

Outside political groups are starting to look at the Minnesota race as Republicans hope to win back control of the U.S. Senate. The U.S. Chamber of Commerce promised aggressive spending on behalf of McFadden even before Tuesday's primary. Democrat-aligned groups have not yet poured much money into Minnesota.

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